1. Field of the Invention
The present invention generally relates to a bone mineral density analysis method and more particularly to a bone mineral density analysis method using a radiation image of an analysis target bone portion.
The invention also relates to a system for implementing the bone mineral density analysis method described above and a computer readable recording medium on which is recorded a program for causing a computer to perform the method.
2. Description of the Related Art
Heretofore, in order to give a diagnosis for osteoporosis and the like, analysis methods that use a radiation image of an analysis target bone portion to obtain a bone mineral density of the bone portion have been known. As one of such bone mineral density analysis methods which can be performed relatively easily, a so-called MD (Microdensitometry) method is known. Basically, in the MD method, radiation generated from a radiation tube is projected simultaneously onto an analysis target bone portion and a reference substance having a plurality of sections with different radiation transmission characteristics to obtain a radiation image representing the bone portion and the reference substance by detecting radiation transmitted through the bone portion and reference substance with an X-ray film or the like, and a bone mineral density of the bone portion is obtained based on a radiation transmission characteristic of a section of the reference substance having the same density as that of the bone portion in the radiation image.
As for the reference substance, an aluminum slope whose thickness varies continuously is generally used and it is often the case that a thickness of the aluminum slope corresponding to the radiation transmission characteristic is defined as the index representing the bone mineral density.
Among the MD methods, a DIP (Digital Image Processing) method in which a radiation detecting body capable of providing a digital image signal representing a radiation image is used and a bone mineral density is obtained by processing the digital image signal is widely known as described, for example, in Japanese Unexamined Patent Publication No. 2006-334046, Domestic Re-publication of PCT International Publication for Patent Applications No. 2008-044439, and Japanese Unexamined Patent Publication No. 2010-200824. The bone mineral density analysis by the DIP method is getting more and more popular as it is easy to operate and requires only a short time.
In the bone mineral density analysis by DIP method described above, if the tube voltage of the radiation tube is different when capturing a radiation image by projecting radiation onto an analysis target bone portion and reference substance, such as the aluminum slope or the like, the analysis may indicate an erroneous result. Consequently, in the bone mineral density analysis by DIP method, the tube voltage of the radiation tube is set to a specific normal tube voltage (e.g., 50 kV) when capturing a radiation image in order to obtain a universal analysis result.
In order to make bone mineral density analysis results to be highly reliable, it is necessary that the tube voltage must be set accurately to the normal tube voltage when capturing a radiation image. Even when the tube voltage is set to the normal tube voltage, however, the actual tube voltage, i.e., the effective tube voltage may sometimes differ from the normal tube voltage due to a temporal change in the characteristics of the radiation tube.
In order to prevent such cases, measures have been taken, such as, measuring the tube voltage of the radiation tube by a tube voltmeter and calibrating such that the setting value of the tube voltage corresponds to the effective value. The tube voltmeter is very expensive, however, and it is undesirable to permanently provide a voltmeter in each medical institution, such as a hospital or the like, from the viewpoint of cost for bone mineral density analysis.
The present invention has been developed in view of the circumstances described above and it is an object of the present invention to provide a bone mineral density analysis method capable of obtaining universal bone mineral density analysis results even if the effective voltage of the radiation tube is different from the normal tube voltage.
It is a further object of the present invention to provide a bone mineral density analysis system for implementing the bone mineral density analysis method and a computer readable recording medium on which is recorded a program for causing a computer to perform the method.